1. Field of the Invention
This invention relates to the field of vanadium recovery, and, in particular, it relates to methods for selectively recovering vanadium from phosphoric acid and from ores, metals, and other sources.
2. Description of the Art
Vanadium is employed in a wide variety of utilities including the manufacture of ferrous and non-ferrous metals, as a target material for x-rays, and for the manufacture of a variety of vanadium compounds many of which, in turn, are employed for the manufacture of catalysts such as hydrocarbon conversion catalysts and catalysts for the manufacture of sulfuric acid. It is not found in its elemental form in nature but is prevalent in more than 65 vanadium-containing minerals and rocks, principle of which are patronite, roscolite, carnotite, and vanadanite. Vanadium is also found in a variety of processed materials and manufactured articles including wet-process phosphoric acid, partially or completely processed ores (including minerals and rocks) and mine tailings (including mining and processing residues), ferrous and non-ferrous vanadium alloys, petroleum code, spent phosphoric acid pickling acids, petroleum, petroleum ash, ferrophosphorous slag from elemental phosphorous manufacture, coal and other sources.
The quantity of vanadium contained in just one source --merchant grade (green) wet-process phosphoric acid--would be sufficient, if recoverable, to supply a major portion, if not all, of the vanadium employed in industry. Wet-process phosphoric acid is typically defined as phosphoric acid which results from the dissolution of phosphate rock (calcium phosphate) in sulfuric acid to form free phosphoric acid and calcium sulfate. The latter, being insoluble, is separated from the phosphoric acid product by filtration to obtain a crude phosphoric acid solution to varying concentration (depending on processing conditions). The crude acid is a highly impure material, generally dark in color, and it contains relatively large amounts of dissolved metals, sulphates and smaller amounts of fluorides, fluorosilicates, and other salts of aluminum, magnesium, iron, uranium, vanadium, and other metals, as well as suspended organic matter.
Wet-process acid typically contains between about 25 and about 52 weight percent phosphorous expressed as P.sub.2 O.sub.5 and about 0.1 to about 2 weight percent vanadium expressed as the metal. Vanadium is usually present in phosphoric acid, including wet-process acid and other phosphoric acid solutions, in a reduced state as the tri-or tetravalent vanadium-phosphate complex. This complex is very difficult to break chemically, and it is even more difficult to selectively recover vanadium from phosphoric acid solutions which contain other dissolved and/or complexed metals and metal salts.
While it is known that vanadium can be precipitated from phosphoric acid by neutralization of the acid to a pH of about 7, such treatment also results in the precipitation of other metal compounds. The vanadium can be recovered from such precipitates only by complex extraction procedures which require the use of relatively expensive organic extractants. One such process described by Hurst et al., in U.S. Pat. No. 3,835,214 involves the extraction of vanadium and/or uranium, while in a reduced valent state, from wet-process phosphoric acid into an organic phase containing certain orthophosphoric acid esters. In another process described by Hurst et al. in U.S. Pat. No. 3,711,591 and by lucid et al. in U.S. Pat. No. 4,212,8949, the vanadium and/or uranium are converted to an oxidized state and are extracted into an organic phase containing certain di-substituted esters of orthophosphoric acid together with a triorganophosphine oxide. The phosphoric acid esters involved in such processes include relatively expensive compounds such as di-2-ethylhexyl)-phosphoric acid combined with trioctylphosphine oxide, and dioctylphenyl phosphoric acid combined with trioctylphosphine oxide. One variation of such processes, which is described by Tebbe in U.S. Pat. No. 4,341,743, involves initially admixing wet-process phosphoric acid containing vanadium, and optionally uranium, with a mixture of a disubstituted ester of orthophosphoric acid and a triorganophosphine oxide as described by Hurst et al. and Lucid et al., supra, followed by gradual addition of hydrogen peroxide to oxidize the vanadium and uranium to vanadium (V) and uranium (VI) in order to increase hydrogen peroxide efficiency. According to Tebbe, supra, the lower valence states of vanadium and uranium are inefficiently extracted into the organic phase, and the conversion of those elements to their higher oxidation states markedly increases extraction efficiency.
Similar problems exist with the extraction of vanadium from acids other than phosphoric acid, since such acids, which contain significant amounts of dissolved vanadium, invariably contain other metals and compounds which complicate attempts to selectively recover vanadium from such solutions.